The research project outlined in this proposal holds the potential for development of one or more radiolabeled Gastrin Releasing Peptide (GRP) receptor-avid radiopharmaceuticals. We propose that these radiopharmaceuticals will be effective site-directed agents, for the treatment and/or diagnosis of patients with localized, as well as metastasized breast cancer. GRP receptors are present on estrogen receptor positive (ER+) and negative (ER-) breast cancer cells found in primary and metastatic tissue. We have previously demonstrated that selected radiolabeled bombesin (BBN) analogs developed in our laboratory target the GRP receptor with high affinity and specificity. Our initial work has resulted in the development of 99mTc- and 111In-BBN analogs which demonstrate high GRP receptor specificity and prolonged retention in vivo in GRP receptor expressing tumors including the ER+, T47D human breast cancer xenograft model. Our research plan focuses on four primary areas: 1) evaluate the diagnostic capabilities of 111In-BBN analogs to target and stage ER+ and ER- human breast tumor xenografts. This will involve assessing the applicability of these diagnostic agents for use in monitoring the course of radiotherapy and chemotherapy treatment; 2) determine the internal radiation dosimetry of 111In, 90Y, 149Pm, and 177Lu-BBN analogs in xenograft models of human breast cancer; 3) evaluate the therapeutic efficacy of 90Y, 149Pm, and 177Lu-BBN analogs in xenograft models of breast cancer. This will involve the determination of a maximum tolerable dose estimate for each isotope-BBN analog evaluated with subsequent single dose and multi-dose radiotherapy trial evaluation; 4) evaluate the synergism of targeted radiotherapy and chemotherapy in the treatment of ER+ and ER-xenograft models of human breast cancer. The experimental approaches used in the proposed research will utilize scintigraphic evaluation of 111In-BBN analog pharmacokinetics and in vivo tumor targeting capabilities with correlative data obtained using a Micro-CT. In vivo breast cancer SCID mouse tumor models (T47D and MDA-MB-231 cell origin), will be the primary assessment tool for analysis of 111In, 90Y, 149Pm, and 177Lu-BBN analog efficacy in therapeutic and diagnostic applications. The approach taken in this proposal will continue our development of a unique class of highly selective diagnostic and therapeutic BBN radiopharmaceuticals that target primary and metastatic breast cancer. Utilizing this selective radiotherapeutic targeting approach, either alone or combined with other chemotherapeutics, may provide new opportunities to manage and treat breast cancer.